Auto-stereoscopic methods present what appear to be three-dimensional imagery to a viewer without the need for special glasses or other impediments (hence “auto” stereo). The use of lens sheets, composed of an array of small lenses, to produce auto-stereoscopic imagery is widely known. A lens sheet typically consists of a closely-packed array of plano-convex lenses. The lenses are generally either spherical in shape (for applications in what are called integral methods) or cylindrical in shape (for applications in what are called lenticular methods). The lens sheet itself is transparent and the rear face, which generally constitutes the focal plane, is effectively flat.
When an integral lens array sheet is coated with, or brought into intimate contact with, a light sensitive material layer (such as a photographic emulsion) at the sheet's focal plane and an exposure is made of an illuminated object or image placed at a close proximity to the lens side of the sheet, each individual lens will record its own unique image of the object or image based on its position relative to the array. In other words, the integral method produces a large number of minute, juxtaposed images behind the lens array on the light sensitive material layer. When realigned in original register with the same, or similar, lens sheet after development of the exposure, a composite spatial reconstruction of the object is re-created in front of the lens array that can be viewed from arbitrary directions within a limited viewing angle.
This method of creating images is generally known as Integral Photography or Integral Imaging and was first proposed in 1908 by physicist Professor Gabriel M. Lippmann. He described a method to record a complete spatial image on a photographic plate, with parallax in all directions, utilizing an array of small spherical lenses to both record and playback the image. In his method, later known as the direct method, an object or scene is recorded directly in front of the lens array. Because of limitations in the resolving power of the individual lenslets, the distance an object could be placed in front of the array was limited, and indeed only objects located a few centimeters from the array where properly re-imaged. Unfortunately, unwanted moiré artifacts were also common in this method because of sub image crossover, which occurs when sub images are not juxtaposed and a sub image formed by one lenslet would cross over to an adjacent lenslet. Further, the method only allowed for objects to be recreated in front of the lens array, in other words, objects that appeared to float only in front of the lens array, not within or behind it.
Herbert E. Ives later improved the technique in 1930, by incorporating a large aperture camera lens (a lens with a diameter wider than the interocular distance between the eyes) to optically suspend a “real” aerial image of an object in front of, within, or behind the lens array. Later known as the indirect method, this allowed for a substantial increase the depth of field and for objects to appear to float behind the lens array, instead of just in front. Ives also proposed the use of a large concave mirror as an alternative to the primary lens.
The general optical principles of the indirect method, using a primary lens, are nearly identical to those of an ordinary camera, with three exceptions. First, the objective lens is typically much larger than a normal camera lens, so chosen to accept a wide field of view of an object. Second, a lens array is placed directly in front of and often coated with the light sensitive emulsion, with the lenslet side facing the objective lens. Third, the “real” object is not brought into focus, instead it is placed relative to the lens screen/material layer in such a manner to recreate the appearance of that object at that position.
The indirect method in integral imaging, which often consists of an optical assembly of compound lenses, allows the location of the aerial “real” image to be adjusted by either adjusting the location of the object, modifying the optical assembly, or adjusting the proximity of the lens array within the focal plane of the camera, all along the z axis of the optical train. In other words, objects could be made to appear floating in front of, at the surface, or inside the lens array, or a combination thereof, simply by making one of these adjustments in a precise manner.
Unfortunately, some form of spherical distortion artifacts are common by virtue of the requirement of a relatively large aperture wide angle primary lenses or concave mirror, and the cameras used in such systems are only capable of imaging relatively small actual objects. The biggest drawback, however, to the photographic integral methods is that the recorded images are pseudoscopic, or depth reversed, where the foreground is the background and vise versa. Several complex photomechanical solutions were later proposed to invert the depth. Known collectively as the “two step” methods, they typically involve a secondary exposure of the original photographic plate through another lens sheet.
Later, a “one step” imaging solution was proposed. The “one step” imaging solution includes presenting a calculated computer generated pseudoscopic image to the lens array that naturally re-inverts the image. The image is formed by moving a series of progressively changing contours of an image, in layers, on a cathode ray tube (“CRT”) screen or by presenting a succession of computer written transparency film masks in front of a high intensity light source along the optical z axis. The result is a fully volumetric computer generated image. The image is recorded through an integral lens array to a light sensitive emulsion. This approach is based on the direct method (it does not incorporate a primary lens or lenses) therefore resulting in a limited depth-of-field and only being able to reproduce objects that appeared to float in front of the lens screen.
In a known one-step indirect method, the virtual object is formed using a high intensity laser that is scanned by a galvanometer directly to the lens screen through a series of optics. In this method, the primary lens or lens screen is moved along the optical z axis as the image is drawn to achieve a fully volumetric image. This is recorded through an integral lens array to a metal based material layer that is generally ablated or altered thermally to form an image. Such a method has several drawbacks in practical field use for digital variable imagery including the length of the optical path and the use of galvanometer laser scanners and associated optics to produce the image. Use of such equipment likely limits the wide spread use of the method in the field, for example, to produce identification cards (at a local Department of Motor Vehicles, for instance), where a compact, high-speed, user-friendly solution is required.
Moreover, known refractive lens based integral methods incorporate commercially available lens arrays, which limit the use of the methods for security-level anti-counterfeiting applications. Such methods further do not contemplate optical designs that result in an optimized focal point to enable higher lens array frequencies beyond commonly available lens arrays. The use of the integral imagery for anti-counterfeiting applications requires the use of a new approach to lenslet design that puts the misuse of the array, even if duplicated, out of the reach of common counterfeiting methods, such as lithographic printing.
A further need exists for imaging to non-uniform surfaces such as anti-tamper wraps on pharmaceutical bottles or on pharmaceuticals themselves. Nearly all efforts to discourage counterfeiting of pharmaceuticals and other counterfeit-prone edible products have been implemented on the outer packaging of products, including micro-printed, tamper-resistant containers and holograms and covert security features on the packaging. Besides identifiable shapes, colors, and embossed features, few anti-counterfeiting security features have been proposed to protect the edible products themselves. The advantage of creating consumer-recognized security features on the pills themselves becomes especially important when considering the practice of dispensing bulk pharmaceuticals, where no other high security features exist.
One known method includes ink-jet printing edible lens array-related imagery, using edible inks, onto edible products, and then molding an edible lens array on the printed image to create a variety of edible, lens array-based effects. Pharmaceuticals and other small, edible products, however, require a very high frequency lens array, typically exceeding 40 lenses per linear centimeter (exceeding 100 lenses per linear inch), which would greatly limit the quality of the imagery using this method, as ink-jet printing resolutions are generally insufficient for such an array. A higher resolution imaging method is therefore required to yield high quality effects, especially when utilized to provide authentication protection.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and/or relative positioning of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the invention. It will further be appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. It will also be understood that the terms and expressions used herein have the ordinary meaning as is accorded to such terms and expressions with respect to their corresponding respective areas of inquiry and study except where specific meanings have otherwise been set forth herein.